«CIPM MRA-D-05 Version 1.6 Measurement comparisons in the context of the CIPM MRA CIPM MRA-D-05 Contents 1. Background 1.1. On the CIPM MRA 2. ...»
Measurement comparisons in the
Measurement comparisons in the context of the CIPM MRA
1.1. On the CIPM MRA
2.1. Key comparison
2.1.1. CIPM key comparison
2.1.2. RMO key comparison
2.1.3. Key comparison reference value
2.1.4. Degrees of equivalence
2.2. Supplementary comparisons
2.3. Pilot studies
3. Registration of comparisons in the KCDB and status report
3.1. Nomenclature of key comparisons
4. CIPM key comparisons
4.1. Participation in CIPM key comparisons
4.2. Initiating a CIPM key comparison
4.3. Organization of a CIPM key comparison
4.4. Technical protocol for a key comparison
4.5. Circulation of transfer standards and customs formalities
4.6. Reporting the results of the measurements
4.7. Report of a CIPM key comparison
5. RMO key comparisons
5.1. Participation in RMO key comparisons
5.2. Organization of RMO key comparisons
5.3. Reports of RMO key
10. Related documents
11. Revision history
Appendix 1 – Flowchart of CIPM and RMO key comparisons
Appendix 2 – Flowchart of subsequent bilateral comparisons
Appendix 3 – Flowchart of supplementary comparisons
1.1. On the CIPM MRA
Paragraph 3 of the CIPM MRA defines the technical basis of the arrangement:
3.1 The technical basis of this arrangement is the set of results obtained in the course of time through key comparisons carried out by the Consultative Committees of the CIPM, the BIPM and the regional metrology organizations (RMOs), and published by the BIPM and maintained in the key comparison database1. Detailed technical provisions are given in the Technical Supplement to this arrangement.
3.2 Key comparisons carried out by Consultative Committees or the BIPM are referred to as CIPM key comparisons; key comparisons carried out by regional metrology organizations are referred to as RMO key comparisons; RMO key comparisons must be linked to the corresponding CIPM key comparisons by means of joint participants. The degree of equivalence derived from an RMO key comparison has the same status as that derived from a CIPM key comparison.
2.1. Key comparison A key comparison is one of the set of comparisons selected by a Consultative Committee to test the principal techniques and methods in the field.
Note: Key comparisons may include comparisons of representations of multiples and sub-multiples of SI base and derived units as well as comparisons of artefacts.
At present the key comparison data base (KCDB) is maintained by the KCDB office of the BIPM
2.1.1. CIPM key comparison A CIPM key comparison is a key comparison executed in the framework of a Consultative Committee or by the BIPM. A CIPM key comparison leads to a key comparison reference value2.
Note: BIPM key comparisons consist of series of successive bilateral comparisons between NMIs and the BIPM performed when the BIPM holds a unique facility (e.g. SIR for activity of radionuclides).
2.1.2. RMO key comparison A RMO key comparison is a key comparison executed in the framework of a regional metrology organization.
2.1.3. Key comparison reference value The key comparison reference value is the reference value resulting from the measurements taken in a CIPM key comparison, accompanied by its uncertainty (normally the standard uncertainty).
Only CIPM key comparisons (carried out by a Consultative Committee or the BIPM) result in a key comparison reference value. For a key comparison carried out by a regional metrology organization (RMO key comparison) the link to the key comparison reference value is obtained by reference to the results from those institutes which have also taken part in the CIPM key comparison.
Note: The method used to determine the key comparison reference value is part of the protocol of the comparison and is agreed by the Consultative Committee or by the appropriate working group to which the Consultative Committee has delegated this task.
2.1.4. Degrees of equivalence The degree of equivalence relative to the key comparison reference value of a measurement standard or of a measurement result is the degree to which the measured value is consistent with the key comparison reference value. This is expressed quantitatively by two
terms: the deviation from the key comparison reference value and the expanded uncertainty of this deviation computed at a 95 % level of confidence (in practice, this is often approximated by using a coverage factor k equal to 2). The “graph of equivalence” shows the degrees of equivalence relative to the key comparison reference value.
The degree of equivalence between two measurement standards or two measurements results (also known as “bilateral degree of equivalence”, or “pair-wise degree of equivalence”) is expressed quantitatively by two terms: the deviation of one measured value with respect to the other, calculated as the difference between their respective deviations from the key comparison reference value and the expanded uncertainty of this deviation computed at a 95 % level of confidence (in practice, this is often approximated by using a coverage factor k equal to 2)3.
The matrix of equivalence consists of the full set of degrees of equivalence. This may be published in the KCDB but in the event that it is not published, the details relating to its calculation are available in the final report.
2.2. Supplementary comparisons A supplementary comparison is a comparison, usually carried out by an RMO to meet specific needs not covered by key comparisons (e.g. regional needs), for instance measurements of specific artefacts, or measurements of parameters not within the “normal” scope of the Consultative Committees.
Consultative Committees may however decide to run a supplementary comparison when there are only few participants capable of measuring the required quantity (none sharing the same RMO), when no link can be made to an RMO comparison or when the distribution of samples to measure is a constraint (for instance: measurements of radioactive matrix reference materials).
In special cases, the CCs may decide that the degrees of equivalence be expressed in relative values, after normalization relatively to the key comparison reference value or the nominal value of the measurand.
2.3. Pilot studies Pilot studies are a third category of comparison normally undertaken to establish measurement parameters for a “new” field or instrument, or as a training exercise. The results of pilot studies alone are not normally considered sufficient support for calibration and measurement capability4 (CMC).
3. Registration of comparisons in the KCDB and status report Registration of comparisons must be made through the BIPM key comparison database office (KCDB Office), using the Key and supplementary comparison registration and progress form. Only key and supplementary comparisons are registered in the KCDB.
Participants who are either i) signatories of the CIPM MRA, or ii) designated institutes for their country through the process of CIPM MRA-D-06, will be listed in the public website of the KCDB for the comparison.
During the course of a comparison that is registered in the KCDB, it is important that up-to-date information on the progress of the comparison be readily available. This implies that the participants, the KCDB Office, and the Consultative Committee (the President, the Executive Secretary, and the working group designated by the Consultative Committee for this task) should be regularly informed by the pilot institute of the status of the comparison.
The progress of a comparison shall be reported to the KCDB office with the same form used for registration. Once the progress of the comparison is reported to the KCDB office, the updated status will be made public on the KCDB website.
3.1. Nomenclature of key comparisons Upon registration with the KCDB Office, each key or supplementary comparison is identified by a unique nomenclature.5 See document CIPM MRA-D-04 Some RMOs also use an internal identifier before the comparison is registered. This identifier may be kept in the KCDB and can be found using the website free form search engine.
The first part of the name identifies the comparison. A second part may be used to identify sub-sets of a particular comparison (see below).
First, the body under the auspices of which the comparison is carried out. This can be:
- Consultative Committee: CC
- Regional metrology organization designated by its acronym: AFRIMETS., APMP., COOMET., EURAMET., SIM., etc.
The dot (.) is added for the BIPM and for the acronym of a regional metrology organization for clarity in reading the nomenclature.
Second, the field of measurement, designated as in the titles of Consultative Committees:
AUV for Acoustics, Ultrasound and Vibration;
Third, which applies only when the comparison is specifically chosen by a given Consultative Committee working group, part of the acronym of the working group
preceded by a dot, for instance:
.RF for the Working Group on Radio Frequencies of the CCEM;
.M for the Working Group on Mass Standards of the CCM;
.P for the Working Group on Pressure of the CCM;
.W for Underwater acoustics.
Fourth, a hyphen (-).
Fifth, a capital letter, K for key comparison, S for supplementary comparison and P for pilot study.
Sixth, a number, generally in the successive order 1, 2, 3, etc.
The second part of the nomenclature may be omitted, but is useful to distinguish between several sub-comparisons of a key or supplementary comparison. It can take any form
but should always be preceded by a dot (.). The most usual cases are:
-.a,.b,.c for several sub-comparisons, corresponding to different ranges of measurements of the same quantity;
-.Xy-αβγ which appears in the field of ionizing radiation for identifying the measurement of a specific radionuclide αβγXy.
-.1,.2,.3 for subsequent bilateral comparisons to a key comparison.
- the year in which the comparison is initially registered in the KCDB.
Dots or hyphens can be added as desirable for clarity in this second part of the nomenclature.
are identified with different numbers, in which case the second part may be kept unchanged.
However, it is possible to keep the same number, in which case changing the second part is mandatory.
4. CIPM key comparisons The Consultative Committees are responsible for choosing the key comparisons. In each field a set of key comparisons is identified which covers a range of standards in order to test the principal techniques in the field.
The procedures used by Consultative Committees for selecting, conducting and evaluating key comparisons, including their detailed technical protocols and periodicity, are
designed to ensure that:
• the comparisons test all the principal techniques in the field;
• the results are clear and unequivocal;
• the results are easy to compare with those of corresponding comparisons carried out subsequently by regional metrology organizations;
• overall, the comparisons are sufficient in range and frequency to demonstrate and maintain equivalence between the laboratories participating in the CIPM MRA.
4.1. Participation in CIPM key comparisons